Coherent Dynamics of Optical Excitation

In this chapter we consider ultra-fast phenomena related to coherently excited (virtual) quasi-particles before the coherence is lost by some interaction. A prominent example is the optical Stark effect which leads to a blue (red) shift of a resonance (now between dressed states) for detuning of the exciting laser below (above) resonant conditions. We describe here this effect for various materials and their quantum-well structures and elaborate the influence of excitonc interaction. We then turn to resonant excitation conditions. Under high excitation of a two-level system two sidebands occur additional to a resonant flourescence line forming the Mollow triplet. The sidebands are determined by the Rabi splitting which corresponds in the time domain to the coherent Rabi oscillations of the system. Besides observation of such Rabi oscillations we will look at its application in coherent control and the state preparation in quantum dots for the emission of indistinguishable photons. Cascaded radiative decay of biexcitons is considered for the generation of entangled photon pairs used in quantum information processing. Finally quantum beats resulting from the coherent superposition of two energy-split states are reviewed. We introduce again important experimental techniques to address the described phenomena: resonance flourescence to study the coherence of resonantly excited states, the Hong-Ou-Mandel to determine the correlation properties of the emitted single photons and four-wave mixing (FWM) which is a standard tool to reveal coherent dynamics. The theoretical description of the coherent phenomena stays on an intuitive level treating quasi-particles by two-level systems.